1. Field of the Invention
The invention relates to a natural circulation indirect type flue gas reheater.
2. Description of the Related Art
As the flue gas from a boiler contains sulfur dioxide, direct discharge thereof tends to produce acid precipitation, causing serious environmental pollution. Limestone (or lime)-gypsum wet desulfurization has been widely used for desulfurization of the flue gas. The limestone (or lime)-gypsum wet desulfurization system often includes a gas-gas heat exchanger (GGH).
The work flow chart of the GGH is shown in FIG. 1. The flue gas prior to entering the desulfurization tower 1 directly exchanges heat with the purified flue gas discharged out of the desulfurization tower via a rotary heat exchanger 4, and the purified flue gas after heat absorption enters the chimney 5. The high temperature section 2 of the flue and the low temperature section 3 (transporting purified flue gas) of the flue are vertically arranged in parallel. In the operation process of the conventional GGH, limited by the structure of the rotary heat exchanger, the sealing performance thereof is not good, so that leakage easily happens, sulfur dioxide escapes during the operation process, the efficiency of desulfurization decreases, even the emission cannot meet the standard. The structure of the rotary heat exchanger is illustrated in FIG. 2. In another aspect, sulfur dioxide contained in the flue gas, as described above, contacts with the water vapor of the flue gas to produce sulfuric acid when the temperature of the flue gas is lower than a certain degree, thereby causing corrosion on the device. Furthermore, after the installation of the GGH, the corrosion on the elements of the GGH and the blockage of the heat exchanger elements decrease the availability of the wet desulfurization system and improve the maintenance cost for the GGH.
Low-temperature electrostatic precipitator (ESP) technology has solved problems of the wet chimney after the desulfurization to a certain degree. Working process of the technology includes adopting a heating medium (generally the water medium) to exchange heat with the flue gas via the heat recovery device and the reheater so as to decrease the operating temperature of the water medium entering the ESP from a normal low temperature state (130-140° C.) to a much lower temperature state (90-100° C.) and increase the temperature of the flue gas after desulfurization from 50° C. to 90° C., thereby utilizing the waste heat of the flue gas and decreasing the energy consumption, improving the dust removal and desulfurization efficiencies, saving the water for desulfurization, and alleviating corrosion on the downstream devices of the ESP. However, the heat medium adopted in the technology requires a heating pump to transport, which results in much increase of the operation cost.
Also, there is an evaporating cooling flue gas reheater for substituting the conventional GGH. In the flue gas reheater, a primary flue gas heat exchanger and a purified flue gas heat exchanger are connected via a vapor chamber, and a vacuum pump is used to control pressures in the heat exchangers to regulate the wall temperature. However, because the primary flue gas heat exchanger is directly connected with the purified flue gas heat exchanger via the vapor chamber, the circulation of the medium cannot go on smoothly. In addition, the wall temperature is controlled using the vacuum pump, during the practical application process, the pressure measurement by the pressure sensor is often inaccurate, so that the control system cannot work effectively, resulting in unstable operation of the whole system.